Have you ever had a dream, Neo, that you were so sure was real? What if you were unable to wake from that dream? - Morpheus (The Matrix).
If you haven't watched "The Matrix" (1999), you really should.
The movie portrays a post-apocalyptic world where smart machines have taken over and built a simulated reality called "The Matrix". Within this simulation, humans are plugged in and experience a computer-generated world, without realizing that their real lives are very different. In the movie, humans are connected to supercomputers through cables that send electrical signals to their brains, making them feel everything happening in The Matrix.
The movie explores the last human city standing, bugs in the system, viruses, and the revolution against the machines. But the main focus is on the idea of the generated dream world.
The movie's central premise is that what we see and feel every day is nothing but the interpretation of the electrical signals received by our brain.
For instance, when your mother bakes a cake, particles enter your nose, and your brain interprets them as the smell of cake. Your tongue touches the cake, and your brain receives a signal that it is delicious and tastes like chocolate.
It is the cooperation of trillions of neurons in your brain that interprets this complex signal.
If we had a monitor between the nose and the brain, we could use it to record the signal sent by the nose and recreate it, along with the other senses.
If we could access all the neurons in the brain through tiny cables, we could send signals to each one, knowing which one is responsible for each sense.
We could send one signal recorded earlier, like the taste signal, to the brain, and it would interpret it as if you were tasting it for real. But if we sent two signals simultaneously, the taste and the smell of chocolate, your brain would make you smell and taste it.
We could even send more signals simultaneously - the two signals of smell and taste, along with one for sight, sound, and even feeling the cake in your hands.
The sent signals act as a video signal recorded earlier of seeing, holding, eating, and even hearing yourself eat a chocolate cake being sent to your brain.
Now you are officially in a "real" dream, where your brain makes you think it is happening without it really happening. Basically a simulation that is indistinguishable from reality.
If you think this is far fetched, think about people with delusional disorders who can't tell what is real and what is not. These people are not faking it; they see and hear things that aren't happening, it is real to them as reality is real to you.
This even happens to mentally healthy people every day; you may look in a side direction quickly and think you saw something, but when you look again you realize it was just your mind playing tricks on you. A simple glitch of a faulty brain signal.
Let's take a step in the sci-fi direction:
Assume we have a powerful monitor to locate and record the most complex brain signals, a powerful computer to analyze signals, access to all brain neurons. Assume we also have an understanding of each neuron's job, and that we can synthesize brain signals (we can send signals to each neuron).
Of couse we don't have all these tools yet. But with research being done on understanding the brain and its signals, we can be optimistic that we will have all the tools in the future, perhaps by the 31 century.
In this future, we could create a dream world with a supercomputer connected to our brain that sends trillions of signals and generates whole scenes.
All the signals needed to simulate a scene as reality exist in the future.
In fact, in the future, computers are so strong they could generate days or years, or even a whole life time worth of scenes. It could run for a long time, allowing us to see, smell, taste, feel, and hear everything that the computer generates as it was our reality.
Now, imagine being born and plugged into this computer, and that the simulation is set to run a dream world similar to the way things were in the early 21 century.
How would you know you're not in that simulation right now? How would you know what is real and what is a powerful simulation?